This invention relates generally to an improved stereolithography apparatus and method for the production of three-dimensional objects in layers, and more specifically, to an improved apparatus and method for recoating a previously-formed cross-section with a layer of stereolithography medium in anticipation of the formation of a next cross-section.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY" have come into use. Basically, stereolithography is a method for automatically building complex three-dimensional parts (e.g., plastic parts) by successively curing a plurality of thin layers of a curable medium (e.g., polymerizable liquid) on top of each other until all of the thin layers are joined together to form a whole part. Each layer is in essence a thin cross-section of the desired three-dimensional object. With this technology, the parts are literally grown from a supply of building medium (e.g., grown from a vat of liquid plastic). This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes. Moreover, complex parts can be made quickly without tooling. Because the system uses a computer to generate the cross-sectional patterns, the system can be readily linked to CAD/CAM systems.
The presently preferred building media are photopolymers that are cured by ultraviolet (UV) light and their curing is fast enough to make them practical model building materials. The liquid that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet laser generates a small intense spot of UV which is moved across the liquid surface with a galvanometer mirror X-Y scanner in a predetermined pattern. The scanner is driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
Other preferred building media includes sinterable powders and powders solidifiable when combined with an appropriately dispensed binder. A typical stereolithography system for use with photopolymers includes a laser scanner, a vat or tank for containing the polymerizable liquid, and an object support platform, which is capable of being raised and lowered in the tank, and a controlling computer. The system is programmed to automatically make a plastic part by forming one thin cross-section at a time and building the desired three-dimensional object up layer by layer.
In typical stereolithographic procedures, a thin layer of viscous curable plastic liquid is applied to a surface which may be a previously cured layer and, after sufficient time has elapsed for the thin layer of polymerizable liquid to smooth out by gravity, a computer controlled beam of radiation is moved across the thin liquid layer to sufficiently cure the plastic liquid so that subsequent layers can be applied thereto. The waiting period for the thin layer to level varies depending on several factors such as the viscosity of the polymerizable liquid, the layer thickness, part geometry and cross-section geometry, and the like.
Typically, the cured layer, which is supported on a vertically movable object support platform, is dipped below the surface of a bath of the viscous polymerizable liquid a distance greater than the desired layer thickness so that liquid flows over the previous cross-section rapidly. Then, the part is raised to a position below the surface of the liquid equal to the desired layer thickness, which forms a bulge of excess material over at least a substantial portion of the previous cross-section. When the surface levels (smooths out), the layer is ready for curing by radiation.
For further details of stereolithography, reference is made to U.S. Pat. Nos. 4,575,330 and 4,999,143 and the following pending U.S. patent applications (all assigned to 3D Systems, Inc., the assignee and applicant for the subject application) which are incorporated herein by reference in their entirety, including appendices attached thereto or material incorporated therein by reference, as if fully set forth:
U.S. Pat. No. 4,104,592 entitled "STEREOLITHOGRAPHIC CURL REDUCTION"; PA1 U.S. patent application Ser. No. 331,644, Hull et al., filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S Pat. No. 5,015,424, entitled APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. Pat. No. 4,999,143 entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. Pat. No. 5,076,974 entitled "METHOD FOR CURING PARTIALLY POLYMERIZED PARTS"; PA1 U.S. Pat. No. 5,658,988, entitled "APPARATUS AND METHOD FOR PROFILING A BEAM"; PA1 U.S. Pat. No. 5,059,021 entitled "APPARATUS AND METHOD FOR CORRECTING FOR DRIFT IN PRODUCTION OF OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. Pat. No. 5,123,734 entitled "APPARATUS AND METHOD FOR CALIBRATING AND NORMALIZING A STEREOLITHOGRAPHIC APPARATUS"; PA1 U.S. patent application Ser. No. 249,399, Almquist et al., filed Sep. 26, 1988, abandoned entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. patent application Ser. No. 427,885, Spence et al., filed Oct. 27, 1989, entitled "STEREOLITHOGRAPHIC APPARATUS"; PA1 U.S. patent application Ser. No. 365,444, Leyden, filed Jun. 12, 1989, entitled "INTEGRATED STEREOLITHOGRAPHY"; PA1 U.S. patent application Ser. No. 265,039, Almquist et al., filed Oct. 31, 1988, abandoned entitled "APPARATUS AND METHOD FOR MEASURING AND CONTROLLING THE LEVEL OF A FLUID;" PA1 U.S. patent application Ser. No. 495,791, Jacobs, filed Mar. 19, 1990, entitled "VIBRATIONALLY ENHANCED STEREOLITHOGRAPHIC RECOATING"; PA1 U.S. patent application Ser. No. 414,200, Hull et al., filed Sep. 28, 1989, entitled "AN APPARATUS AND METHOD FOR FORMING A SUBSTANTIALLY FLAT WORKING SURFACE"; PA1 U.S. patent application Ser. No. 415,168, Hull et al., filed Sep. 29, 1989, entitled "METHODS OF COATING STEREOLITHOGRAPHIC PARTS" PA1 U.S. patent application Ser. No. 429,911, Spence et al., filed Oct. 27, 1989, entitled "IMPROVED SYSTEM FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY EMPLOYING VARIOUS PENETRATION DEPTHS AND BEAM PROFILE"; PA1 U.S. patent application Ser. No. 428,492, Vorgitch et al., filed Oct. 27, 1989, abandoned in favor of U.S. patent application Ser. No. 824,819, entitled "RAPID AND ACCURATE PRODUCTION OF STEREOLITHOGRAPHIC PARTS"; PA1 U.S. patent application Ser. No. 429,435, Smalley et al., filed Oct. 30, 1989, entitled "IMPROVED STEREOLITHOGRAPHIC CONSTRUCTION TECHNIQUES"; PA1 International Application No. PCT/US89/04096, Almquist et al., filed Sep. 26, 1989, entitled "RECOATING OF STEREOLITHOGRAPHIC LAYERS"; and PA1 U.S. patent application Ser. No. 415,314, Jacobs et al., filed Sep. 29, 1989, entitled "IMPROVED STEREOLITHOGRAPHIC POST-CURING." PA1 U.S. patent application Ser. No. 516,145, abandoned in favor of U.S. patent application Ser. No. 07/702,031 entitled "IMPROVED STEREOLITHOGRAPHIC CONSTRUCTION TECHNIQUES."
Previous stereolithographic doctor blades provide means to reduce the cycle time for forming each layer of plastic, but without providing a capability to vary parameters associated with the layer formation process to the specific part geometry at hand. In some cases, therefore, these blades will not give optimal results, especially when the part has a certain geometry including trapped volumes of unformed material, which allow unformed material being swept in front of the blade to flow back underneath the blade to disrupt the layer formation process, or large flat areas, where the shear force exerted by the blade on the unformed material may cause a problem known as scoopout, again, with the result of disrupting the layer formation process. Also, previous blade designs are relatively thin, and sometimes suffers from a flutter and twist problem, whereby the unsupported end of the blade deforms when the blade is sweeping away a bulge of excess material. This further disrupts the formation of a substantially uniform layer. The thin cross-sectional width also contributes to the flowback problem, mentioned earlier, over large trapped volumes.
Moreover, the previous blade designs were rectangular in cross-section and contacted the uncured material over their full cross-sectional width. The contact of the blade with the unformed material may induce a shear force on the material beneath the blade, which may be acceptable as long as the blade is not travelling over a previous object cross-section. However, when the blade begin travelling over the previous cross-section, so that only a thin "channel" of material separates the blade from the previous cross-section, the shear force may actually induce a lift force on the part, causing it in some cases to lift up, and even strike the blade. Another problem is that a small bulge of material may form on the trailing edge of the blade, which the blade may deposit over the leading edge of the part when it travels over the part. This deposit, if allowed to build up at each cross-section, can result in a substantial distortion of the part.
It is also difficult to adjust the blade gap with the previous blades, which gap is the distance between the blade and the working surface of the material. However, it is desirable to be able to adjust the blade gap, since too large a gap may result in substantial movement of the platform through the working surface, resulting in working surface disruptions, while too small a gap may position the blade during sweeping too close to the working surface which may further contribute to surface disruptions. Presently, setting the blade clearance requires the loosening and tightening of bolts, which may exert a torque on the blade, sufficient to cause the blade to tilt or warp relative to the working surface, which may be detrimental to uniform layer formation.
Measuring and controlling the level of the working fluid in a stereolithographic apparatus is also desirable with layer formation using a blade. Detecting the level of the working fluid is important to the layer recoating process so that the previous cross-section can be lowered sufficiently below the working surface to ensure an adequate formation of excess material over the previous cross-section.
Many scientific experiments and industrial applications require the measurement of the level of a fluid. The term "level of a fluid" as used here means the height of the surface of a fluid in a gravitational field or other accelerated frame of reference. This surface may be the top or even the bottom of the fluid (if the fluid is floating on another fluid). The fluid may be the ocean, the gasoline in the tank of an automobile or a liquid chemical in a test tube, among many possibilities. Various means have been adopted over the years to measure the levels of such fluids, including dip sticks, lines painted on the side of pilings, marks on the side of test tubes, floats, reflected light beams, and the like. A need exists, however, for an apparatus which can very precisely and reliably measure the level of a fluid in a stereolithographic apparatus. Apparatus of this sort is particularly useful if coupled with a level maintenance means such as a plunger, a diaphragm, a lifting and lowering means to vertically translate a container of building material, or controls for a pump in order to maintain the level of the fluid at any desired height.
In particular, stereolithographic machines require very precise control of the level of a working fluid. U.S. Pat. No. 4,575,330 to Charles W. Hull, mentioned earlier, discloses apparatus for production of three dimensional objects by stereolithography. The working fluid used in stereolithographic apparatus is usually a photopolymer liquid curable by the application of ultraviolet (U.V.) light. As noted in U.S. Pat. No. 4,575,330, the level of the working fluid in the preferred embodiment must be maintained at a constant level so that the beam of U.V. light will remain sharply in focus on a fixed plane.
The overall intensity and intensity profile ("beam profile") of the beam of U.V. light at the surface of the liquid photopolymer will determine, in cooperation with other factors (such as the characteristics of the liquid photopolymer and the length of time the beam remains in a single spot), the depth and profile of the photopolymer that is cured or polymerized by exposure to the beam. The beam profile will vary with the level of the liquid photopolymer, because the beam is focussed to have a known profile at a predetermined level of the liquid photopolymer. If the liquid photopolymer has a level different from the predetermined one, the difference in the beam profile will change the width of the cured photopolymer and its depth from the depth and width planned.
Furthermore, if the level of the liquid photopolymer is higher than the predetermined level, the depth of the cured photopolymer may not be sufficient to reach to and adhere with the previously cured layer, with detrimental consequences for the structural integrity of the object being formed. If the level is lower, then the new layer will be thinner than planned, which is detrimental to the accuracy of reproduction of the object.
The level of the liquid photopolymer must be maintained despite the shrinkage caused by curing the liquid photopolymer, heating, evaporation, and the like. In early versions of stereolithographic apparatus, this level was maintained by providing a spillway. The level of the liquid photopolymer rose to and slightly above (because of surface tension) the spillway. A spillway, however, does not control the level of the liquid photopolymer with sufficient precision to make possible the finer resolution of parts made by a stereolithographic apparatus. Previous level-detection apparatus provided a more precise means of measuring the level of a fluid through the use of bi-cell detectors to detect the change in position of a reflected beam, reflected off the surface of material in a side tank. However, in many instances, it is desirable to be able to detect level changes in the main tank, rather than in a side tank, since the level in the side tank may not provide a good indication of the level in the main tank. This may be the case if the building material is floated on an immiscible liquid as described in Ser. No. 365,444. A problem with measuring level changes in the main tank is that bubbles or other surface descriptions may form from the passage of the part and/or part platform through the working surface, and the previous apparatus is sensitive to these surface descriptions in the main tank.
Accordingly, an object of the subject invention is to provide means to tailor the layer formation process to specific part geometries. Another object is to provide a sweeping member used in layer formation which reduces or eliminates material flowback and scoopout, and leading edge material deposit, and which induces less lift forces on the previously-formed portion of the part. Another object is to provide means for level detection which does not require a side tank, and which is less sensitive to surface disruptions on the working surface.
Additional objects and advantages will be set forth in the description which follows or will be apparent to those of skill in the art who practice the invention.